Polyester films, in particular, polyester films primarily containing polyethylene terephthalate, have excellent mechanical properties, thermal properties, electrical properties, surface properties, optical properties, heat resistance, and chemical resistance. They are thus used in a wide range of applications including magnetic recording media, industrial materials, and packaging materials. Since polyester films have low resistance to impact and flex pinhole, biaxially stretched nylon films are widely used in packaging material applications. However, nylon films, which have high moisture absorption and coefficient of moisture expansion, require special attention during preservation or processing and are difficult to work by vapor deposition. In order to increase heat resistance, printability, stiffness, and dimensional stability, nylon films are frequently used with polyester films bonded thereto.
Proposals for rendering impact and flex pinhole resistance to polyester films include a method for obtaining a flexible polyester film by copolymerizing polyethylene terephthalate with other components, such as long-chain aliphatic dicarboxylic acid, e.g., dimer acid (Japanese Unexamined Patent Application Publication No. 6-79776); and a method for obtaining a flexible polyester film by blending modified polybutylene terephthalate into polyethylene terephthalate (Japanese Unexamined Patent Application Publication No. 2001-11213). Such flexible films no longer have the inherent advantage of PET films, i.e., high elastic modulus, and tend to require higher costs since expensive comonomers are used. Moreover, conventional flexible films have low heat resistance and are sticky. Thus, troubles, such as clinging, frequently occur during the film making process or film working process.